Ultrasound testing is usually based on the transmission and the reflection of ultrasound waves within the material of the piece to be tested, and the analysis of the echoes detected in combination with the waves emitted.
Generally speaking, the verification of the calibration of such a non-destructive testing system, prior to the testing of a piece, is carried out with the aid of a reference (or standard) piece which is fabricated using the same material and with the same process as the piece to be tested.
Such reference pieces are gauge blocks whose geometry is known, controlled and regularly checked. They have, in general, various thicknesses in order to be able to create an amplitude-distance correction of the CAD type, and holes with a flat bottom in order to verify the detection.
Generally speaking, the reference pieces are used to carry out:
(i) a verification prior to each scanning of the ultrasound configuration (namely of an amplitude-distance correction);
(ii) a verification of detection minima. These characteristics may be influenced by the ultrasound properties, such as the frequency, the shape of the acoustic beam, the bandwidth, and the scanning means mechanics of the non-destructive testing system; and
(iii) a verification to ensure that the non-destructive testing system exhibits a behavior which can be repeated over time (by comparing mappings of the same reference piece over time).
However, this method of verification of the calibration of the non-destructive testing system has a drawback associated with the necessity to use such reference pieces. Indeed, the reference pieces are very costly. Moreover, their use in the verification of the calibration requires:
(i) the fabrication of many reference pieces;
(ii) their periodic validation; and
(iii) the fabrication of new reference pieces notably in the case of wear or of damage.